Fix NotInParallel Order property not being respected for class-level attributes

[Copilot]

The NotInParallel attribute's Order property was not being respected when applied to classes, causing tests to execute in incorrect order based on dependency graph topological sorting instead of the specified order.

Problem

When using NotInParallel with different Order values on classes, tests were executing in the wrong sequence:

[NotInParallel(Order = 2)]
public sealed class Test1
{
    [Test]
    public async Task TestCase1()
    {
        Console.WriteLine("Test 1");
    }
}

[NotInParallel(Order = 1)]
public sealed class Test2
{
    [Test]
    public Task TestCase2()
    {
        Console.WriteLine("Test 2");
        return Task.CompletedTask;
    }
}

Expected output (v0.25.21):

Test 2
Test 1

Actual output (v0.52.8):

Test 1
Test 2

Root Cause

The issue was in TestScheduler.cs where the ExecuteNotInParallelTestsAsync and ExecuteKeyedNotInParallelTestsAsync methods were sorting tests by the ExecutionPlan's topological order instead of respecting the NotInParallel attribute's Order property. While the priority queue correctly sorted tests by TestPriority (which includes the constraint order), this correct ordering was being overridden by a secondary sort based on ExecutionPlan order.

Solution

Modified the sorting logic in both methods to:

1. Primary sort: Use the NotInParallel Order property from TestPriority
2. Secondary sort: Use ExecutionPlan order only for dependency resolution when NotInParallel orders are equal

This preserves the intended execution order while maintaining dependency ordering capabilities for tests with the same NotInParallel order value.

Verification

Manual testing confirms the fix works correctly:

• Tests now execute in the proper order: Test 2 (Order 1), Test 1 (Order 2), Test 3 (Order 3)
• The change is minimal and surgical, affecting only the sorting logic
• Dependency resolution is preserved as a secondary sort criteria

Fixes #2779.

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